1. Field of the Invention
Exemplary aspects of the present invention generally relate to a transfer device and an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a transfer device including a belt-type transfer member and an image forming apparatus including the transfer device.
2. Description of the Background Art
Conventionally, there is known a transfer device employed in, but not limited to, image forming apparatuses, such as copiers, printers, facsimile machines, and multifunctional systems including at least two of these functions, in which an endless looped belt is employed as an intermediate transfer member for transferring images.
Typically, a color image forming apparatus is equipped with a plurality of cylindrical photoreceptors serving as image bearing members, one for each color, and a belt-type intermediate transfer member (hereinafter referred to as an intermediate transfer belt). The intermediate transfer belt is disposed across from and in contact with the photoreceptors which rotate at a certain speed, and rotates at the same peripheral speed as that of the photoreceptors.
Such a color image forming apparatus equipped with the intermediate transfer belt includes also a developing device, a primary transfer device, and a secondary transfer device. The developing device develops latent images of different colors formed on the photoreceptors into toner images. The primary transfer device transfers overlappingly and sequentially the toner images formed on the photoreceptors-onto the intermediate transfer belt, thereby foaming a composite color toner image thereon. Then, the secondary transfer device transfers the composite color toner image from the intermediate transfer belt onto a transfer material, for example, a recording medium, thereby ultimately forming a color image.
As the composite toner image is transferred from the intermediate transfer belt to the transfer material, it is not always the case that the toner of the composite toner image is transferred completely onto the transfer material. That is, undesirably, some residual toner remains on the intermediate transfer belt. Thus, a cleaning device, often constructed of a cleaning blade made of rubber or resin, is provided to clean the residual toner from the intermediate transfer belt. However, removal of the residual toner by the cleaning blade is becoming more difficult in recent years as toner consisting of very fine spherical particles increasingly comes to be used to satisfy growing market demand for the production of images of ever-higher quality.
A conventional method of facilitating removal of residual toner employs a lubricant which is applied on the surface of the intermediate transfer belt to reduce adherence between the intermediate transfer belt and the residual toner. Generally, the lubricant is applied on the intermediate transfer belt using an application member, for example, a brush roller. The brush roller consists of a metal core around which a fabric with brush bristles is attached, thereby rubbing the lubricant retained on the brush bristles onto the intermediate transfer belt.
Application of the lubricant is best accomplished in the absence of residual toner. Thus, the lubricant is usually applied to the intermediate transfer belt after the intermediate transfer belt is cleaned by the cleaning device.
In such a conventional method, a roller (hereinafter referred to as an opposing roller) is disposed opposite the brush roller through the intermediate transfer belt (that is, on a side of the belt opposite the side on which the brush roller is disposed) to press the spanned surface of the intermediate transfer belt against the tip of the brush roller so that the tip of the brush roller can reliably contact the surface of the intermediate transfer belt while preventing rippling of the intermediate transfer belt as well. For example, the amount of the tip of the brush bristles of the brush roller engaging the belt surface is approximately 1 mm.
Although advantageous, there is a drawback to this configuration in that, because the tip of the brush roller engages the belt surface to a depth of approximately 1 mm, the brush roller contacts not only the portion of the intermediate transfer belt contacting the opposing roller, but also other areas of the intermediate transfer belt not contacting the opposing roller, for example, areas adjacent to both ends of the contact area of the intermediate transfer belt with the opposing roller.
As a result, when pressed by the tip of the brush roller, the portion of the intermediate transfer belt where the opposing roller does not contact the belt deforms under pressure from the brush. (By contrast, when pressed by the tip of the brush roller, the portion of the intermediate transfer belt contacting the opposing roller is prevented from being deformed.) When the tip of the brush roller separates from the intermediate transfer belt, the intermediate transfer belt flexes back to its original shape.
As the brush roller repeatedly contacts and separates from the intermediate transfer belt, flexing the intermediate transfer belt continuously, rippling occurs in the intermediate transfer belt, thus resulting in undesirable vibration of the intermediate transfer belt. Such vibration hinders the intermediate transfer belt from reliably contacting the photoreceptors at their proper transfer positions, thus resulting in improper transfer of toner images. This improper transfer of the toner images may show up as banding in the resulting output image. In particular, when printing a halftone image, horizontal lines of narrow pitch appear in the image.
While prevention of vibration of the intermediate transfer belt is of critical importance, in order to achieve desirable transfer performance it is generally desirable to wind around and stretch the intermediate transfer belt between two rollers so that the intermediate transfer belt can reliably contact the photoreceptors and primary transfer rollers serving as the primary transfer device at a constant pressure. However, increasing demand for making an image forming apparatus as compact as possible does not always allow for such a configuration.
If the intermediate transfer belt is wound around the photoreceptors or the primary transfer rollers, the tension of the intermediate transfer belt causes the contact pressure between the photoreceptors and the primary transfer rollers to vary. This also results in improper transfer of the toner images.
In view of the above, a device that can reduce, if not prevent entirely, vibration of the intermediate transfer belt is required while also making the transfer device as compact as possible.